There is a significant number of patents which describe different constructions of cryosurgical probes and catheters. These patents aims to solve some of the problems, which are common to the cryosurgical probes and catheters of the prior art.
One of these problems is the construction of relatively cheap and simple probes or catheters with high reliability and sufficiently effective thermal insulation of their lateral non-operating walls. Moreover, cryosurgical catheters should have high flexibility, especially, when they are used for cardiac procedures. In addition, the closed distal end (cryotip) of such probe or catheter must provide in many cases high specific freezing capacity at sufficiently low-temperatures.
Analysis of United States patents related to this field shows, that the structure of the proposed probes and catheters intended for cryosurgery does not satisfy the above-mentioned requirements.
For example, U.S. Pat. No. 3,971,383 proposes a cryogenic surgical instrument with a coaxial assembly of flexible tubes, wherein the inner tube is connected to a supply of cryogenic liquid, and the space between the outer wall of the inner tube and the inner wall of an intermediate tube forms a return line for evaporated cryogenic liquid, which is vented to the atmosphere. The space between the outermost coaxial tube and the intermediate tube contains a gas, such as normal butane, for providing thermal insulation of the inner and intermediate lumens.
U.S. Pat. No. 5,716,353 describes a probe for cryosurgery which consists of three tubes: an inner tube for supplying a cryogenic refrigerant to a cryotip positioned at the distal end of an outer jacket tube, and an intermediate tube situated concentrically around the inner tube. The channel between the inner and intermediate tubes serves as venting path for venting cryogenic refrigerant from the freezing zone. This construction is simple, but it does not provide sufficient thermal insulation as required in the construction of cryogenic catheters. Consequently, it may cause over-heating of the vented cryogenic refrigerant, as well as over-cooling of tissues adjacent to the intermediate section of the catheter.
U.S. Pat. No. 5,573,532 describes a cryosurgical instrument which comprises tubes for cryogenic fluid supply and for the return of cryogenic fluid vapors, wherein these tubes are concentric and the return tube is sealed with a cryotip. Vacuum insulation of the return lumen is also proposed. However, this construction is relatively expensive and has low reliability. In addiiton, the proposed vacuum insulation limits the flexibility of the probe, especially, when it is very long, as in the case of catheter implementations.
U.S. Pat. No. 5,674,218 describes a cryosurgical instrument, a system and method of cryosurgery. According to this patent a cryogenic liquid (preferably, liquid nitrogen) is initially sub-cooled below its normal boiling point and in that state supplied into the open proximal end of the internal supply line. The outer lumen of the cryosurgical instrument is provided with active vacuum insulation.
Obviously, this construction cannot provide high flexibility and therefore cannot be used as the basis for construction of a catheter for use in cryosurgery.
U.S. Pat. No. 5,254,116 describes a cryocatheter with a set of vent holes in the lateral wall of a central feeding lumen, wherein sub-cooled liquid nitrogen is delivered into the central feeding lumen as a cryogenic liquid. This construction, however does not ensure proper thermal insulation of the cryocatheter.